A re-read of the Response confirms you are correct — he does claim the note is signed with a nickname to hide her identity. And BdlR is such a standup guy, he even blacks out fake names.
I speculate that W8 is thanking Sybrina for helping her develop the talking points so she would know what to say to a big scary lawyer, and for solving her desire to stay out of public involvement by arranging the phone interview scheme (oops...jokes on her). None of that need be nefarious on W8’s part, but would seem to raise enough reasonable doubt to sink W8’s testimony.
I have a feeling that Witness 8 wishes she had never known the Martins.
If events went down as that letter suggests, and she learns within a day or two that her friend is dead, she ... goes mute? That doesn't make sense. Her friend is murdered, she'd make an effort to finger the killer. But she didn't.
Which suggests to me, strongly, that things did not go down as she describes. If she knows Martin started the fight, then going mute makes sense.
Now she's trapped in a very big public spectacle. If she recants, the long knives will be out for her. Snitches get stitches. She'll bluster her way through, perhaps making obvious lies, but sticking to them. I don't think her credibility (or lack thereof) is relevant as Judge Nelson makes an immunity call. The jury is apt to be hung, and Zimmerman can go around the trial merry-go-round again.